I downloaded a bunch of mods: unit line changes, unit replacements, etc. I went to try them out in an official campaign, and only one (XP bank mod) appeared in the dropdown checkbox menu for all of the campaigns. They all appear when I create a local game for multiplayer, and they are all installed and updated when I check my add-on list. I tried uninstalling the XP bank mod. I have tried restarting both Wesnoth itself and my computer. I have checked the forums for the same problem, and all I can find is new players who have installed an mp scenario or some other thing unaware that it is not a user made campaign. I think the installed mods may have appeared at first and then disappeared after I started a campaign. I think I may have encountered the same problem on an older version, though that was a while back. Below is my info as required by the forum.

The Battle for Wesnoth version 1.14.4
Running on Microsoft Windows 10 (10.0.17134)

the mod designers/creators may not be aware of their mod only working in MP. For example I made couple of mods, but it's first time I hear that the mods can be "designed" or "not designed" for multipplayer only... I have no idea how to make a mod appear in checkbox for campaign and i have no idea if my mods support it (would need to go read about it, if I was interested at this time of my life.. which i'm not unfortunately as i have no time for any modifications). Is it some new 1.14 thing? or...?

my advice for you (tafm1803) personally then would be to find out who created these mods and request them in forum with private message or find their posts/threads or whatever else way you can find, just basically ask them to modify their mod to be compatible for single player campaigns or whatever you need exactly... if somebody asked me for such a change, I most probably would do it in most circumstances.. so don't be shy, just ask them.

the mod designers/creators may not be aware of their mod only working in MP. For example I made couple of mods, but it's first time I hear that the mods can be "designed" or "not designed" for multipplayer only... I have no idea how to make a mod appear in checkbox for campaign and i have no idea if my mods support it (would need to go read about it, if I was interested at this time of my life.. which i'm not unfortunately as i have no time for any modifications). Is it some new 1.14 thing? or...?

Yes, sp modifications are new in 1.14. Whether a modification is available in sp and/or mp is determined by [modification] type attribute (possible values being sp, mp, hybrid).

the most important thing to make your mod work in singleplayere is to not put it inside #ifdef MULTIPLAYER

Scenario with Robots SP scenario (1.11/1.12), allows you to build your units with components, PYR No preperation turn 1.12 mp-mod that allows you to select your units immideately after the game begins.